Consider a tricycle. Start with the trike at rest, with one pedal straight up and the other straight down. If you push forward on the down pedal, which way will the trike go? Here is a nice statement of this problem with a couple of possible arguments for which way it should go. Jim Mandes and Sterling Thomas from our Spring 2003 121 class tried it out. Here are videos of their results, along with their explanation. Thanks Jim and Sterling.